The subjective sense that time moves faster as we get older is a universal one, and over the years scientists have proffered a number of different explanations as to why this happens. A professor of mechanical engineering at Duke University is suggesting a new and strange hypothesis to explain the phenomena, and it has to do with our aging brains.

One of the common psychological explanations behind our sense of time moving faster with age is that the more familiar the perceptual information around us is, the less attention we pay to it. Children, for example, are constantly perceiving new events and environments, using significantly more brain power to process day-to-day information. As we get older, the novelty of our reality slowly tapers off, leaving one with the sense that time is passing more rapidly.

Adrian Bejan, a mechanical engineer at Duke University, has taken this idea and offered a more solid, physical explanation to underpin the phenomena. While we certainly may have processed more information when we were young, giving us the sense of time moving slower, Bejan he claims this to be the result of younger brains being able to identify and integrate mental images at a much more rapid pace.

"People are often amazed at how much they remember from days that seemed to last forever in their youth," says Bejan. "It's not that their experiences were much deeper or more meaningful, it's just that they were being processed in rapid fire."

Bejan's idea is that physical features of our brain that degrade with age underpin our sense of time speeding up. For example, our saccadic frequency is known to decline as we age. This is our ability to perceive single mental images, and studies in infants have revealed younger eyes move around a scene much faster than adults. Bejan suggests this shows younger minds acquiring and integrating more information faster than older minds, and it is this higher load of perceptual data that results in a subjective sense of time moving slower while young and faster when old.

"The human mind senses time changing when the perceived images change," says Bejan. "The present is different from the past because the mental viewing has changed, not because somebody's clock rings. Days seemed to last longer in your youth because the young mind receives more images during one day than the same mind in old age."

Bejan's proposal is undeniably compelling, presenting a neurological mechanism that could reasonably explain the subjective perception of time speeding up with age. However, this purely physical mechanism doesn't entirely explain the seemingly consistent and exponential increase in the speed of time passing from year to year as we get older.

The logarithmic hypothesis fills this gap, suggesting time perception is relative to the proportion of time we have lived. So proportionally, a year to a 10 year old feels much longer than a year to a 50 year old. As Christian Yates, a mathematical biologist from the University of Bath, explains, the perceived experience of time from the age of 10 to 20 is the same proportionally as from 40 to 80.

"To a 10-year-old, a year is only 10% of their life, (making for a slightly more tolerable wait), and to a 20-year-old it is only 5%," Yates explains. "On the logarithmic scale, for a 20-year-old to experience the same proportional increase in age that a two-year-old experiences between birthdays, they would have to wait until they turned 30. Given this view point it's not surprising that time appears to accelerate as we grow older."

All of this certainly leaves us with the unsurprising conclusion that time is complicated, and our perception of it, even more so. Bejan's new idea may be somewhat accurate but it surely is only one piece in the bigger puzzle that is our subjective experience of time.

The article was published in the journal European Review.